Short Coherence Time Research Articles

Statistical properties of backscattered semiconductor laser radiation with different degrees of coherence

This paper compares the statistical properties of the backscatter intensity in a single-mode optical fibre for semiconductor lasers with a high and a low degree of coherence. We demonstrate that, when short probe pulses are used, shorter than the coherence times of the lasers, the statistical properties of the backscatter intensity obtained with the two lasers are identical, and the intensity distribution over an ensemble of independent fibre sections has an exponential form for both. With increasing probe pulse duration, the backscatter intensity distribution obtained with the shorter coherence time laser approaches a Gaussian one, whereas the distribution function obtained with the high-coherence laser remains nearly exponential. Reflectograms of a coherent reflectometer that relies on the detection of backscattered radiation have the highest contrast for an exponential backscatter intensity distribution over an ensemble of independent fibre sections. The reflectometer then probably has the highest sensitivity to external influences. This leads us to conclude that, when short probe pulses areused, which ensure high spatial resolution (10 m and better), one can use a laser with a short coherence time, equal to the pulse duration.

Sequential measurement-based quantum computing with memories

We introduce a general scheme for sequential one-way quantum computation where static systems with long-living quantum coherence (memories) interact with moving systems that may possess very short coherence times. Both the generation of the cluster state needed for the computation and its consumption by measurements are carried out simultaneously. As a consequence, effective clusters of one spatial dimension fewer than in the standard approach are sufficient for computation. In particular, universal computation requires only a one-dimensional array of memories. The scheme applies to discrete-variable systems of any dimension as well as to continuous-variable ones, and both are treated equivalently under the light of local complementation of graphs. In this way our formalism introduces a general framework that encompasses and generalizes in a unified manner some previous system-dependent proposals. The procedure is intrinsically well-suited for implementations with atom-photon interfaces.

LOCALIZING SAGITTARIUS A* AND M87 ON MICROARCSECOND SCALES WITH MILLIMETER VERY LONG BASELINE INTERFEROMETRY

ABSTRACT With the advent of the Event Horizon Telescope (EHT), a millimeter/submillimeter very long baseline interferometer (VLBI), it has become possible to image a handful of black holes with sub-horizon resolutions. However, these images do not translate into microarcsecond absolute positions due to the lack of absolute phase information when an external phase reference is not used. Due to the short atmospheric coherence time at these wavelengths, nodding between the source and phase reference is impractical. However, here we suggest an alternative scheme which makes use of the fact that many of the VLBI stations within the EHT are arrays in their own right. With this we show that it should be possible to absolutely position the supermassive black holes at the centers of the Milky Way (Sgr A*) and M87 relative to nearby objects with precisions of roughly 1 μas. This is sufficient to detect the perturbations to Sgr A*'s position resulting from interactions with the stars and stellar-mass black holes in the Galactic cusp on year timescales, and severely constrain the astrophysically relevant parameter space for an orbiting intermediate-mass black hole, implicated in some mechanisms for producing the young massive stars in the Galactic center. For M87, it allows the registering of millimeter images, in which the black hole may be identified by its silhouette against nearby emission, and existing larger-scale radio images, eliminating present ambiguities in the nature of the radio core and inclination, opening angle, and source of the radio jet.

Stellar scintillation in the short exposure regime and atmospheric coherence time evaluation

Aims. Accurately measuring the atmospheric coherence time is still a significant problem despite a variety of applicable methods. The Multi-Aperture Scintillation Sensor (MASS) designed for the vertical profiling of optical turbulence also provides a measurements of coherence time, but its results were found to be biased. Hence there is a need for a more robust method to determine τ0.

Emission characteristics of a highly correlated system of a quantum dot coupled to two distinct micropillar cavity modes

The emission characteristics of a system of one quantum dot (QD) nonresonantly coupled to two distinct detuned modes of a surrounding microcavity have been investigated, revealing strong interconnection in terms of photon anti-correlation in QD-mode and mode-mode cross correlations. Temperature-dependent lifetime measurements demonstrate that the QD emission dynamics is transferred to the mode channels through the process of nonresonant coupling, whereas the short coherence time of mode emission stays unperturbed by the QD-mode detuning. Power-dependent studies under pure-resonant QD excitation have traced the strong emitter-mode coupling to both an excited and the fundamental micropillar mode with spectral detunings of up to 3.7 meV.

Spin in CdTe/ZnTe Quantum Dot: Its Potential for Information Storage

Inventions of transistor [1] and integrated circuit [2] established a close link between semiconductor physics and information processing. Rapid development of the field, aptly described by Moore’s law [3], based on optimization of growth and processing techniques as well as on progress in designing complex CPU circuits. Quantum phenomena, inevitable in nanoscale systems, have been considered a limitation to further development of classical electronics according to Moore’s law. Simultaneously, quantum mechanics was recognized as an opportunity to extend the bounds of classical computational paradigm through an idea of quantum computer, described by Deutsch in 1985 [4]. Physical realization of proper building blocks for quantum computer — qubits conform to certain requirements [5] — exhibits major experimental difficulty. Many different approaches were proposed, including trapped ions [6], cavity QED systems [7], liquid state NMR [8], gated quantum dots [9], superconductors [10], crystal lattice impurities [11], and others. Among all these possible solutions, solid-state systems are particularly promising due to relative ease of qubit manipulation, e.g., by electric, optical or magnetic fields. Prospects of future integration of qubits with existing classical electronics also constitute a significant advantage. Drawbacks of the solid state-based systems are related mainly to short coherence times limited by the interaction with the environment. The interaction with the surrounding crystal matrix may be suppressed by e.g. constraining otherwise mobile carriers inside potential well of a quantum dot. This paper presents a selection of our studies of spins in self-assembled CdTe/ZnTe quantum dots from the point of view of quantum information processing. We will discuss three basic processes related to a single quantum dot used as a qubit, that is read-out of encoded information, writing (initialization) the qubit state and evolution of the qubit state which provides means to manipulate it. 2. Samples

Deriving Ocean Surface Drift Using Multiple SAR Sensors

Tracking and monitoring ocean features which have short coherent time periods from sequential satellite images requires that the images have both very high spatial resolutions and short temporal sampling intervals (i.e., repeated cycles). Satellite images from a single sensor in a polar-orbiting satellite usually cannot meet these requirements since high spatial resolution of the sensor may result in relatively long temporal sampling interval and vice versa, such as the case of Synthetic Aperture Radar (SAR). This paper presents a new multi-sensor approach to overcome the long temporal sampling interval associated with a single SAR sensor while taking advantage of high spatial resolution of SAR images for the application of ocean feature tracking.Currently, there are two SAR sensors on different satellites, the European Remote Sensing Satellite-2 (ERS-2) and the ENVIronment SATellite (ENVISAT), having acquisition time offset around 28 minutes with almost exactly the same path.That is, ERS-2 is following ENVISAT with a 28-minutes delay, which is a good time-scale for ocean mesoscale feature tracking.A pair of SAR images from ERS-2 and ENVISAT collected on April 27, 2005 has been chosen to track ocean surface features by using wavelet analysis. As demonstrated in the case studies, this technique is robust and capable to derive ocean surface drift near an oil slick and around a big eddy in the South China Sea (SCS).

Temporal Coherence of MgO Based Magnetic Tunnel Junction Spin Torque Oscillators

Single-shot, time-resolved measurements are presented to investigate the temporal coherence of the microwave emission for MgO based magnetic tunnel junction spin torque oscillators. The time-domain data reveal that the steady state regime obtained from frequency-domain analysis can be subdivided into two regimes as a function of spin polarized current amplitude. According to these two regimes, two mechanisms that limit the temporal coherence are identified. At low current, extinctions of the steady state oscillations lead to a very short coherence time on the order of a few nanoseconds, while at higher current, the extinctions vanish and the coherence time saturates around 40 ns. As an important result it is shown that the latter is limited by frequency fluctuations. Quenching these frequency fluctuations suggests an intrinsic linewidth that is by a factor of 20 below the one of the free running oscillator.

Optimizing the retrieval efficiency of stored light pulses

We study the retrieval efficiency of stored light pulses based on electromagnetically induced transparency in multiple simultaneously driven Lambda-systems. The light pulses are stored in coherences between different Zeeman states of laser-cooled atoms. When the stray magnetic field from the environment is minimized by compensation coils we observed a smaller retrieved probe pulse amplitude than for a small externally applied magnetic field, i.e., a seemingly shorter coherence time. We identify this effect as a beating of several coherences due to a very small uncompensated dc magnetic stray field. By intentionally applying a small magnetic field larger than this stray field we were able to increase the retrieved probe pulse amplitude up to five-fold to the value determined by the true coherence time of our system.

Dual update-rate carrier tracking technique for new generation global navigation satellite system signals in dynamic environments

With the upcoming of modernised global positioning system (GPS) and Galileo, the new generation global navigation satellite systems (GNSSs) will be able to provide the users a better positioning performance. An important modification on most of the new generation GNSS signal structures is the availability of the pilot channel, which stands for an additional measurement obtained; moreover, a pure phase-locked loop (PLL) can be used in carrier-phase tracking loop. Utilising this characteristic, the authors present a dual update-rate PLL for modernised GNSS signals to deal with the well-known dilemma that the usage of long coherent integral time can help in reducing the noise from thermal effects, whereas the usage of short coherent integral time can permit the tracking loop to suffer more dynamics stress. By processing data and pilot channels with different coherent integral times, different discriminators, and then performing the fusion of those two different update-rate measurements in a Kalman-filter-based loop filter, this loop offers several advantages over the traditional methods in dynamic and low carrier-to-noise ratio (CNR) environments. Simulation results demonstrate that the proposed dual update-rate PLL shows good performance in both dynamic and low CNR environments.

Limited Role of Spectra in Dynamo Theory: Coherent versus Random Dynamos

We discuss the importance of phase information and coherence times in determining the dynamo properties of turbulent flows. We compare the kinematic dynamo properties of three flows with the same energy spectrum. The first flow is dominated by coherent structures with nontrivial phase information and long eddy coherence times, the second has random phases and long-coherence time, the third has nontrivial phase information, but short coherence time. We demonstrate that the first flow is the most efficient kinematic dynamo, owing to the presence of sustained stretching and constructive folding. We argue that these results place limitations on the possible inferences of the dynamo properties of flows from the use of spectra alone, and that the role of coherent structures must always be accounted for.

Temporal coherence of mode arrivals

Temporal coherencies are compared for individual pulse arrivals of 100 and 800 Hz centered broadband signals propagating through identical shallow channels during periods of low and high internal wave energy. All 100 Hz modes are clean, distinct in time, and remarkably coherent (t(coh)>1 h). A near continuum of modes is observed for the 800 Hz signal with much shorter coherence times. Higher order modes are less coherent. Internal wave scattering appears to dominate the 100 Hz signals, whereas bottom scattering also randomizes the 800 Hz signal. Observations are consistent with similar measurements for this and other sites for intermediate frequencies.

Generation of time-bin-entangled photons without temporal postselection

We report on the implementation of an interferometric scheme that allows the generation of photon pairs entangled in the time-energy degree of freedom. This scheme does not require any kind of temporal postselection on the generated pairs and can be used even with lasers with short coherence time.

Ultrafast hybrid plasmonics

We review our recent studies of electromagnetic coupling and associated temporal dynamics of molecular excitations with plasmonic resonances supported by either localized or extended planar geometries. We focus on coherent interactions between plasmon resonances and molecular excitations, which are experimentally challenging due to the very short (∼10–100 fs) coherence times of plasmons. Recent experimental results and theoretical analysis for observing and controlling coherences between molecular excitations and plasmonic polarizations are shown. Advances will explore new directions in ultrafast coherent control of molecular excited states and energy dissipation processes, as well as ultrafast addressing and switching in plasmonics-based circuit architectures.

Maximum-Likelihood Detection of Orthogonal Space-Time Block Coded OFDM in Unknown Block Fading Channels

For orthogonal space-time block coded orthogonal frequency division multiplexing (OSTBC-OFDM) systems, many of the existing blind detection and channel estimation methods rely on the assumption that the channel is static for many OSTBC-OFDM blocks. This paper considers the blind (semiblind) maximum-likelihood (ML) detection problem of OSTBC-OFDM with a single OSTBC-OFDM block only. The merit of such an investigation is the ability to accommodate channels with shorter coherence time. We examine both the implementation and identifiability issues, with a focus on BPSK or QPSK constellations. In the implementation, we propose reduced-complexity detection schemes using subchannel grouping. In the identifiability analysis, we show that the proposed schemes can ensure a probability one identifiability condition using very few number of pilots. For example, the proposed semiblind detection scheme only requires a single pilot code for unique data identification; while the conventional pilot-based channel estimation method requires L pilots where L denotes the channel length. Our simulation results demonstrate that the proposed schemes can provide performance close to that of their nonblind counterparts.

An Optimal Antenna Assignment Strategy for Information Raining

This paper addresses the antenna assignment problem for distributed multiple-antenna architecture that enables wireless communication between onboard and ground in subway and railway. We propose a class of algorithms that match the assignment pattern to the large scale fading. Such algorithms are not constrained by short coherence time inherited in subway and railway. We will then derive an optimal antenna assignment strategy employing maximum ratio combining. Simulation results will be provided to demonstrate the advantages of the proposed strategy, which can also be applied to the recently proposed information raining system.

Training-Based MIMO Systems—Part I: Performance Comparison

In this paper, we make a performance comparison between two different training-based schemes for multiple-input multiple-output (MIMO) channel estimation. The two schemes are the conventional time-multiplexed pilot (CP) scheme and the more recently suggested superimposed pilot (SIP) scheme. Unlike previous comparisons found in the literature, which are mostly based on estimation error performances, the performance comparison in this paper is made by deriving and comparing the maximum data rate (or rather a tight lower bound on the maximum mutual information) achieved by each scheme. By using the maximum mutual information criterion, for each training scheme, we can optimally allocate the time and power spent on transmission of training and data sequences. Once the system parameters (time and power) are tuned to give optimal performance, we can compare their respective maximum data rate. The theory is applied to a blockwise flat-fading MIMO channel, and it is found that in certain scenarios (such as many receive antennas and/or short channel coherence times), it is beneficial to use the SIP. In other scenarios, the SIP scheme suffers from a higher estimation error, and its gain over the CP scheme is often lost.

Increasing the coherence time of Bose-Einstein-condensate interferometers with optical control of dynamics

Atom interferometers using Bose-Einstein condensate that is confined in a waveguide and manipulated by optical pulses have been limited by their short coherence times. We present a theoretical model that offers a physically simple explanation for the loss of contrast and propose the method for increasing the fringe contrast by recombining the atoms at a different time. A simple, quantitatively accurate, analytical expression for the optimized recombination time is presented and used to place limits on the physical parameters for which the contrast may be recovered.

Theory of fourfold interference with photon pairs from spatially separated sources

We present a theory for fourfold quantum interference of photons generated from independent spontaneous parametric down-conversion processes. Closed-form expressions for fourfold quantum interference patterns and visibility are found. The theoretical result for fourfold quantum interference patterns is in good agreement with experimental data reported. Detailed numerical calculations for the dependence of fourfold quantum interference visibility on experimentally controllable parameters are carried out. It is found out that higher visibility can be achieved for small biphoton width, short pump pulse coherence time, and narrow bandwidth of spectral filters. The optimal condition for obtaining at the same time higher fourfold interference visibility and intensity is also discussed.

Prism coupling of ultrashort light pulses into waveguides

We discuss coupling of ultrashort light pulses into waveguides by use of a prism waveguide coupler configuration. Theoretical analysis indicates that an extra loss induced by the short coherence times of ultrashort pulses, which has a strong effect on the reflected light and the optimum coupling condition, appears in the waveguide. Numerical simulations show that the reflectance strongly depends on the coherence times of ultrashort pulses. A method for realizing optimum coupling by compensating for the extra loss is proposed as well in this paper. A preliminary experiment of employing ultrashort pulses with different coherence times was carried out, and good agreement between theory and experiment was obtained.